The invention relates to the production of materials suitable for disposal from of aqueous suspensions of solids, liquids and usually gases (also known as "sludges"), and in particular to the production of disposable solids from sludges produced in municipal and/or industrial waste treatment plants which treat such waste with biological agents.
Municipal and industrial waste treatment plants produce large volumes of sludges containing solids, entrained gases and residual water. Typically, such sludges contain 99 weight percent water or more. Some portions of the sludge are heavier than water and tend to sink; other portions (often containing entrained gases) tend to float; and any water separating out of the overall sludge usually contains high concentrations of solids. These sludges are costly to transport and to properly dispose, due in large measure to the high concentration of water. Various methods have been proposed to remove some of this water and produce a higher solids content concentrate, however, the methods proposed to date have been costly (e.g., evaporation of the water) and/or relatively ineffective (e.g., filtration) in achieving significant reductions in the water content. For example, filtration and/or pressing of the sludge commonly produces a semi-solid, messy filter cake which still contains more than 90 weight percent water. Disposal of sludges, and even the somewhat more concentrated material (i.e., from the evaporation, filtration and/or pressing methods) in landfills usually results in additional problems, e.g., the release of malodorous vapors and the gradual seepage of this water out of the disposal pile over a long period of time, contaminating ground water if not properly contained and treated. Part of the problem appears to result from the tendency of biologically treated materials to contain surface active agents, and the biological agents themselves, which tend to releasably bind water to the solids, thus rendering pre-disposal separation of the water more difficult. Prior methods also produce relatively poor quality water from the separation process, in that the solids content of such water tends to be unacceptably high.
Accordingly, there is a need for an improved method for producing disposable materials from biologically treated sludges, which method economically and effectively reduces the water content of the separated solids, and provides a cleaner separated water, in order to mitigate the above described problems.